<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #d1{
            width: 200px;
            height: 200px;
            border: 10px inset red;
            border-radius: 200px;
        }
        #d2{
            width: 100px;
            height: 100px;
            border: 1px solid red;
            /*给元素添加内边距默认影响元素的宽高*/
            padding-left: 50px;
            padding-top: 50px;
            /*添加下面样式后 内边距不再影响宽高*/
            box-sizing: border-box;
        }
        body{margin: 0}
        p,ul,h3{margin: 0}
        input{border: 0}
        ul{padding: 0}
    </style>
</head>
<body>
<input type="text" placeholder="请输入用户名">
<p>p</p>
<h3>h3</h3>
<ul>
    <li>aaa</li>
    <li>bbb</li>
</ul>
<div id="d1">边框测试</div>
<div id="d2">内边距</div>
</body>
</html>